This invention generally relates to the field of monitoring a sound source for determining its operating condition, particularly relates to the fields of machinery condition monitoring, acoustics, and digital signal processing, and specifically relates to the real-time digital filtering of an acoustical signal to obtain its power spectrum, and the comparison of the power spectrum with a previously determined baseline spectrum as a means of detecting developing machinery faults.
There has been commercial activity in the field of machinery condition monitoring for at least 25 years, almost all of it based on either periodic or continuous measurement of machine vibration. Acoustic monitoring has rarely been used for detecting machinery faults, even though sound and vibration are closely related. A rotating or reciprocating machine, for example, produces dynamic forces (forces which are rapidly changing functions of time) which cause various parts of the machine to vibrate. These vibrations also cause sound to be radiated from the machine. The relationship between the dynamic forces acting within a machine and the sound radiating from the machine is complex. The vibration spectrum (the displacement amplitude as a function of frequency) depends on the measurement location on the machine, as well as the orientation of the vibration transducer with respect to the axis of rotation of the machine. The sound spectrum (the acoustic power as a function of frequency) depends on the orientation of the microphone with respect to the machine, the directional characteristics of the microphone, and the acoustical characteristics of the surrounding objects and structures. The point of origin of a vibration component may not be an efficient radiator of sound; nevertheless, it is still possible to hear this vibration component if it is transmitted to another part of the machine which is mechanically resonant at that frequency. The design of the machine, and especially the damping characteristics of the materials used, greatly affects the intensity and spectral distribution of the radiated sound.
A machine which is in good condition and functioning properly will have a certain vibration spectrum, which in turn will generate a certain sound spectrum, that is, an acoustic signature which can be used as a reference or baseline. In general, the vibration spectrum and the sound spectrum are not the same; in fact, they may be quite different. But, if the condition of the machine deteriorates, or if there is a sudden failure, the vibration spectrum, and therefore the sound spectrum, will change. The deteriorating machine condition can be detected by continuously monitoring the sound coming from the machine, computing the power spectrum, and comparing the power spectrum to the baseline spectrum stored in memory. If the real-time power spectrum deviates from the baseline spectrum by more than a predetermined amount, an alarm can be activated, along with automatic shutdown of the monitored machine, if desired.
There are many types of machine faults that could be detected by such a monitor; for example, rotating imbalance, reciprocating imbalance, misaligned or bent shafts, damaged rolling element bearings, damaged journal bearings, damaged or worn gears, broken drive belts or chains, mechanical looseness, jamming, overloading, friction, windage, impacts, explosions, and escaping air, water, or steam. An acoustic monitor could also provide protection for non-rotating equipment such as boilers, electrical transformers, and flow processes.
The art and science of vibration-based machinery condition monitoring is highly developed, and there are many commercially available products for measuring vibration, and for collecting, storing, analyzing, and displaying vibration data. In recent years, there has been a significant increase in activity in this field because of the widespread availability of digital signal processing (DSP) hardware such as DSP microcomputers. These are high speed single-chip computers which incorporate a high degree of operational parallelism and which are designed to implement computationally intense DSP algorithms such as the fast Fourier transform (FFT), widely used to compute the power spectrum of a vibration signal. Vibration analysis techniques have been developed to detect and diagnose specific machine faults, using commercially available hardware and software tools. The usual approach is to measure vibration with an accelerometer which is in direct contact with the machine being monitored or studied, and then process the resulting signal with an instrument known as a dynamic signal analyzer (DSA). This equipment is expensive, the placement and orientation of accelerometers on the machine can be critical, and skilled personnel are required to operate the DSA and correctly interpret the resulting vibration spectra.
With regard to the early detection of machinery problems, in many cases the first indication of trouble is the sound that a machine makes. In fact, it may be argued that acoustic monitoring (by human observers) is the oldest form of machinery condition monitoring in existence. Experienced machine operators or plant maintenance personnel can often recognize that a machine is in distress because they are familiar with what the machine sounds like when it is operating normally. An acoustic monitor could, in effect, replace human observers in situations where machinery is operating in remote, inaccessible, or hazardous locations, or any other situation where machinery requires continuous monitoring. The acoustic monitor according to the teachings of the present invention is intended to be affordable, dependable, easy-to-use, and easy-to-install and has, as its purpose, machinery protection rather than machinery fault diagnosis or testing. Once the user has been alerted to the fact that machinery is in distress, more sophisticated equipment can be used to diagnose the specific problem. It is not necessary to continuously monitor the machinery with costly vibration-based instrumentation.
At the present time, there is only one commercially available product known to be capable of continuous acoustic monitoring of industrial processes and equipment: the Model 261 Sound Level Detector/Controller, manufactured by Quest Electronics of Oconomowoc, Wis. This product is essentially a sound level measuring instrument with an output relay having an adjustable threshold calibrated in decibels (dB). It measures the root-mean-square (RMS) sound pressure level (SPL) sensed by a microphone and actuates a relay if the threshold setting is exceeded. Other than providing the A and c frequency weighting commonly used for sound level measurements, this product does not perform any type of filtering or spectral analysis. It is a broadband instrument which simply measures the combined effect of all the frequency components of a signal. Primarily intended for industrial hygiene purposes (noise control and warning), it can also be used to provide an alarm signal or automatically shut down a machine if the sound pressure level exceeds the threshold setting.
An acoustic monitor according to the teachings of the present invention is a self-contained system which detects faults in the operating condition by continuously analyzing the sound produced by the sound source being monitored and comparing the resulting power spectrum to a previously recorded xe2x80x9cacoustic signaturexe2x80x9d used as a baseline. In the preferred form, the acoustic monitor performs real-time {fraction (1/12)}th octave digital bandpass filtering over an eight octave range (midband frequencies of 33.108 Hertz to 8,000 Hertz) and computes the acoustic power output, in decibels, of each of the resulting 96 bandpass filters. Fractional octave bandpass filtering produces a constant percentage bandwidth analysis, that is, the bandwidth of each bandpass filter is a constant percentage of its midband frequency. In the case of a {fraction (1/12)}th octave bandpass filter, the bandwidth is always 5.78 percent of the midband frequency. This type of spectrum analysis is widely used in the field of acoustics, as opposed to the constant bandwidth FFT analysis preferred for vibration measurements. In terms of signal processing, the acoustic monitor according to the teachings of the present invention functions in exactly the same way as an instrument known as a digital filter analyzer. The digital filters conform to American National Standard S1.11-1986 xe2x80x9cSpecification for Octave-Band and Fractional-Octave-Band Analog and Digital Filtersxe2x80x9d. The use of digital filters in the acoustic monitor according to the teachings of the present invention, as opposed to analog filters, is highly desirable for three reasons: (1) Analog implementation of the 96 bandpass filters described herein would require a very large number of precision resistors, capacitors, and operational amplifiers, (2) Component aging and drift would cause the filter characteristics to change over time and temperature, and (3) It is easy to control, simulate, and modify, if necessary, the characteristics of digital filters implemented in software.
The acoustic monitor according to the teachings of the present invention has two modes of operation: learn and operate. Before protection can be provided, the monitor must be placed in a learn mode for a period of time so it can xe2x80x9clearnxe2x80x9d what the sound source sounds like when the sound source is known to be operating properly. The monitor can remain in the learn mode for a few minutes, several hours, or even days, but it must be a long enough time for the acoustic monitor to experience all of the sounds which normally occur in the environment in which the sound source is located. During the learn mode, the maximum and minimum acoustic power output from each one of the 96 bandpass filters is continuously maintained and updated in data memory as the acoustic signature of the sound source being monitored. In this manner, the alarm limits are automatically established, without requiring the judgment and experience of a skilled operator. A copy of the acoustic signature is also maintained in non-volatile memory (NVM) which preserves the data whenever the acoustic monitor is powered down. While in the learn mode, the acoustic signature is written to NVM every ten minutes. It is also written to NVM whenever the front panel mode switch is changed from LEARN to OPERATE. The NVM copy of the acoustic signature cannot be continuously updated because the electrically erasable programmable read-only memory (EEPROM) used for this purpose in the preferred form typically has an endurance of no more than one million write cycles. Thus, an electrical power interruption during the learn mode would cause, at most, ten minutes of data to be lost. Operator intervention is required to continue in the learn mode after power is restored. This is to prevent the acoustic monitor from powering up unexpectedly in the learn mode and corrupting an acoustic signature which is already stored in NVM.
During the operate mode, the acoustic monitor of the preferred form of the present invention continuously compares the real-time filter outputs with the acoustic signature previously stored during the learn mode and activates a panel lamp and relay if the output of any of the 96 bandpass filters deviates from the upper or lower decibel limits of the acoustic signature by more than the setting of the corresponding front panel sensitivity selector switch. There are two alarm settings: warning and danger. The warning and danger levels, in decibels, can be set independently and can be individually configured for either latching or non-latching alarm operation, using front panel switches. A latching alarm remains active until the clear button is pressed or a valid signal is received at the remote clear terminal, even if the machinery or similar sound source returns to normal operation. A non-latching alarm is automatically deactivated if the machinery or similar sound source returns to normal. During non-latching operation, both alarms employ hysteresis to prevent relay chatter when slowly changing sounds are encountered.
In the preferred form, the acoustic power output of each bandpass filter is computed by squaring its output and time-averaging the result, because the energy in a wave is proportional to the square of its amplitude. The response time of the averaging filters can be adjusted from 1 to 1000 seconds, using a front panel selector switch. The response time is defined as the time required for the filter outputs to settle to within one percent of their final value after a step change in acoustic power. Note that this is not necessarily equal to the length of time it takes for the acoustic monitor to respond to an operating condition fault. It is merely another way of specifying the transient response of a first-order system (response time=4.605 time constants). Selecting the appropriate response time for the application will enable the acoustic monitor to respond to an operating condition fault within a reasonable length of time, while ignoring short-term background noise events.
The acoustic monitor according to the teachings of the present invention allows the user to record and utilize up to five acoustic signatures. This multiple acoustic signature capability is designed for monitoring applications that involve more than one acoustical xe2x80x9cphase of operationxe2x80x9d. That is, the sound radiating from a sound source may not be continuous in nature, but may be characterized as having several distinct regions of operation, each having its own acoustic signature. For example, an automated test stand which uses sound to detect product defects could perform up to five types of tests, each producing a different acoustic signature. However, the acoustic monitor cannot automatically recognize which test is being performed or what phase of operation a sound source is engaged in because, without additional information, the acoustic monitor could interpret the normal sound during one phase of operation as an abnormal condition of another phase of operation. During the operate mode, the acoustic monitor must receive a command from either an operator or a host controller, such as a programmable logic controller (PLC), to change acoustic signatures. The command can be given manually, using the front panel clear button, or by a host controller which sends an appropriately timed pulse through the monitor""s remote clear terminal.
During both the learn and operate modes, the acoustic monitor according to the teachings of the present invention continuously computes the real-time power spectrum of the sound sensed by the microphone. The power spectrum of a signal is valuable information that is widely used in the field of acoustics, and the ability to view it in real time represents a powerful capability which traditionally has been very expensive. The acoustic monitor according to the teachings of the present invention has terminals which allow the user to view a graphical display of the following important data with the aid of an ordinary oscilloscope: (1) The real-time power spectrum of the acoustic signal, (2) The upper decibel limit of the stored acoustic signature, and (3) The lower decibel limit of the stored acoustic signature. A trigger signal is also provided by the acoustic monitor to facilitate external triggering of the oscilloscope. In each case, the display is in the form of a step graph which simultaneously shows the time-averaged acoustic power outputs of all 96 digital bandpass filters used to measure the power spectrum of the signal. Even the most basic oscilloscope has two channels, allowing simultaneous display of the upper and lower decibel (dB) limits which constitute the acoustic signature. All three graphical displays incorporate horizontal (time) and vertical (voltage) markers which allow the user to easily adjust out any inaccuracies in oscilloscope calibration. The horizontal scale is calibrated in octaves (a logarithmic measure of frequency) and the vertical scale is calibrated in decibels (a logarithmic measure of acoustic power), consistent with practices in the field of acoustics. In its preferred form, the acoustic monitor according to the teachings of the present invention consists of two components: the control unit and the microphone unit. The control unit contains the power supply, digital signal processor, EPROM boot memory, non-volatile memory, A/D converter, D/A converter, lamp/relay driver, switches, indicator lamps, relays, and screw terminals, packaged in a DIN rail-mountable enclosure which meets international safety standards. There are relay outputs that can be connected to an annunciator panel and others that can be used to control the machinery or process being monitored.
The microphone unit contains a microphone, along with analog signal conditioning circuitry, housed in a compact, rugged enclosure suitable for use in an industrial environment. The control unit and microphone unit are connected by a 4-conductor shielded cable which supplies DC power to the signal conditioning circuitry inside the microphone unit, is while sending the amplified microphone signal in differential form back to the control unit for digital processing. This configuration yields the maximum signal-to-noise ratio in electrically noisy industrial environments.
A main object of the invention is to provide a new and improved acoustic monitor for monitoring and evaluating sounds emitted by a sound source wherein the acoustic monitor is the type which computes the spectrum of the monitored sound, and wherein the acoustic monitor has a learning mode wherein a sound spectrum of a monitored sound source is computed and stored as a signature spectrum and has an operating mode wherein the sound spectrum of a monitored sound source is computed continuously and compared with the stored signature spectrum, and any deviations therefrom of predetermined values are taken note of as a basis for possible corrective action.
This and further objects and advantages of the present invention will become clearer in light of the following detailed description of an illustrative embodiment of this invention described in connection with the drawings.